(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of improved Critical Dimension control for various E-beam and laser beam pattern exposure systems.
(2) Description of the Prior Art
The creation of semiconductor devices is critically dependent on the formation of numerous patterns that form device features while other patterns provide the means of interconnecting these device features. Photolithography has long been the mainstay technology that has been used for the creation of these patterns, the art of photolithography depends on transferring patterns that are contained in a mask onto a target surface whereby the most frequently used target surface is a layer of photoresist that is sensitive to impact of photo-electric energy. By placing a source of light behind the mask and exposing the layer of photoresist via the interception of the mask that contains the to be created pattern, the light that is provided by the light source only partially penetrates the mask and consequently only partially impacts the surface of the layer of photoresist. The chemical composition and molecular structure of the photoresist is changed due to the impact of the photo energy such that the impacted or non-impacted photoresist, dependant on whether the photoresist is positive or negative type photoresist, can be removed thereby leaving the pattern that is present in the exposure mask imprinted and reflected in the layer of photoresist. The remaining photoresist can then be used to shield an underlying layer of material, such as conductive materials, to create a desired pattern in these materials. It is well known in the art that for instance patterns of interconnect metal lines can be created in this manner.
The methods of creating images onto a target surface have been expanded to where not only visible or ultraviolet light energy is used for the exposure of the layer of photoresist (optical lithography using proximity or projection exposure) but where other sources of energy, such as electron beam projection, ion bombardment and X-rays can also be used as the source of energy.
Electron beam exposure and laser beam scanning have initially been applied for the creation of masks that are used for photolithographic exposures. The process of creating patterns whereby a photolithographic mask is used depends on accurate and easy alignment of the mask with underlying target surfaces, this in order to enable accurate exposure in a readily reproducible and repeatable manner. Since the image that is contained in the mask is projected onto a target surface, the method whereby the mask image is projected onto the target surface is frequently referred to as projection printing. These photolithographic technologies have advanced from earlier methods of proximity printing using a negative photoresist to methods of projection printing using a positive photoresist and a wafer stepper.
The methods of photolithographic are simple to apply but suffer from defect formation for methods where the mask is in direct contact with the wafer onto which an image is to be created. This contact is cause for high mask damage resulting in the requirement for frequent mask replacement. The major advantage of direct contact printing is that it offers high resolution, for shrinking device features contact printing becomes less attractive due to among others the impact that the (relatively larger) thickness of the layer of photoresist has on image resolution and critical dimensions of pattern images. Proximity printing offers the advantage of decreased mask damage, this advantage however is obtained at the cost of reduced image resolution.
Two of the more important parameters that apply to image creation are the Depth Of Focus (DOF) that can be achieved using a particular technology and the therewith corresponding Numerical Aperture (NA). For high feature resolution it is required that the DOF parameter is as large as possible, which implies that NA must be relatively large since the DOF is inversely proportional to (NA)2. There is however a compromise that must be made between the values of DOF and NA. For narrow line width, NA must be large since the line width is inversely proportional to the NA. This would lead to the conclusion that a (very) large value of NA leads to dense feature or line patterns. A relatively large value for NA however decreases the value of DOF to the point where image resolution suffers. This then leads to the conclusion that an optimum (relatively high) value for NA must be selected such that the DOF is still acceptable, bringing small feature size in balance with acceptable feature resolution. The wavelength of the light source that is used to create the pattern is also of importance whereby DOF is directly proportional to this wavelength while the aperture width is also directly proportional to the wavelength. Because of this dependency, it is clear that one of the approaches that can be used for improved line resolution is the use of smaller wavelength energy sources. Electron lithography makes use of this relation in using smaller wavelength at relatively high source energy. E-beam apparatus makes use of high-energy electrons to directly write a pattern onto a target surface. This leads to two concerns: the electrons that make up the E-beam must be propagated in a narrow and densely controlled beam while the sequential process of writing the pattern consumes a relatively large amount of time that is required to complete the writing of the pattern. The electrons, once they reach the target surface, may also be prone to scatter in a manner that is not conducive to creating a pattern of sub-micron dimensions.
A typical hardware and software configuration that makes up an E-beam system is rather elaborate and requires a considerable amount of software support to create image patterns with the capabilities of fast and error free patterns generation, pattern modification, and the like. Due to the nature of an E-beam configuration, the supporting hardware contains a number of components that are designed for electron beam control (such as the electron gun, which creates the electron beam) and a magnetic configuration that controls the electrons after release by the electron gun and before reaching the target surface. The control of the electron beam, once the beam has been emitted by the electron gun, takes place in an electron beam control column that contains such items of control as one or more lenses for the focusing of the electron beam in addition to one or more aperture masks to restrict the flow of electrons. The main operational parameters of an E-beam apparatus are the accelerating voltage that is applied to the stream of electrons, typically between about 10 and 50 KeV, and the cross section or size of the electron beam, typically between about 0.05 to 3 um. The E-beam apparatus thereby can be of a continuous write nature or it can be of a step and repeat nature containing various optical constructions. The performance parameters of an E-beam system are the feature size resolution, typically less than 0.1 um and the overlay accuracy, typically between about 0.005 and 0.4 um.
E-beam systems differ in the method that is used to project a desired pattern onto a target surface, two methods are essentially used for this purpose, that is raster scanning and vector scanning.
For the raster scanning approach, both the (scanning) E-beam and the target surface move with respect to each other such that the E-beam sweeps in an Y-direction while the underlying target surface moves in an X-direction, whereby the X and the Y direction perpendicularly intercept. The direction of the E-beam during its sweep in the Y-direction is controlled and made to follow the movement of the underlying target surface so that, during one complete sweep of the E-beam across the target surface, the vertical sweep of the E-beam addresses the same X-axis component of the underlying target surface. During the sweep of the E-beam, the stream of electrons is controlled by being interrupted and released in accordance with the pattern density that is required to be written on the target surface. It can be seen that, in this manner, the E-beam can access every point of the target surface and can imprint every point with an amount of electron beam energy that is under control of the E-beam apparatus.
Vector scanning uses an approach whereby a desired pattern in a target surface is decomposed into sub-exposures that are individually exposed by the E-beam onto the target surface. For each element of exposure, the E-beam is positioned on the target surface where that element is to be created after which the E- beam performs the exposure for that element. Immediately after this exposure is completed, the E-beam is directed towards the next element that needs to be exposed, the process continues in this manner in a repetitive fashion. It is not strictly required that sequential elements that are exposed constitute identical elements with identical E-beam exposure for the consecutive elements. E-beam exposure data can be fed to the E-beam control apparatus for each element that is exposed by the E-beam whereby however it is of value to consider, for reasons of throughput, that it may be of benefit the expose identical elements in a sequential stream of exposure. A number of vector scanning machines move the target surface in a step-and-repeat manner, allowing the exposure of larger surface areas. Positioning of the E-beam that is required for the step-and-repeat process does not have to be very accurate since the positioning of the target surface, which is monitored by a laser interferometer, can readily be indicated to the E-beam apparatus, which can then compensate small differences between actual and desired positioning of the target surface.
Further refinements have been introduced in the E-beam exposure approach that are aimed at improving image projecting capabilities of the E-beam system. One of these refinements uses a reduction in the number of exposures that need to be performed in order to create an image by controlling both the shape and the size of the electron beam. By shaping an electron beam that is rectangular in cross section but whereby the size of the cross section can be varied, the size of the exposure that is performed at one instance of exposure can be varied. Successive exposures are made whereby each of the successive exposures addresses portions of an image that needs to be created, by additive (superimposing on the target surface) exposures the actual image is created. This variable shape E-beam system uses two deflectors to control E-beam positioning and overlay. It is clear that this approach leads to rather complex system configurations, a complexity that only gets further emphasized when the variable shape approach is further combined with adjustable positioning of the target surface.
Critical for methods of exposure when using E-beam apparatus is the method that is used to measure positioning of the E-beam with respect to other components of the system, most notably the target surface. Extreme accuracy is required for this exposure, this positioning capability is further implemented and supported by software functions that control other sub-sections of the E-beam apparatus that have an influence on the E-beam reaching the target surface. The method of the invention addresses this concern of accurate positioning.
E-beam technology further uses methods of creating patterns and images that are created by electron proximity and projection printing. Using these latter methods, a mask is applied whereby the distance between the mask and the target surface is about 1 mm. The E-beam is scanned over the entire surface of the chip-sized mask, openings in the mask allow the impinging electrons to penetrate the mask and subsequently strike the target surface. The masks that are applied using this method are relatively small resulting in a step-and-repeat process of exposure. This method creates detached opaque features (where they are required for proper exposure of a target surface) by using two complementary half-masks whereby alignment between these masks is performed using reference marks that are provided for this purpose on the surface of each chip.
With decreasing device feature size, it is increasingly difficult to use E-beam technology to create for instance very dense patterns of interconnect lines. It has already been stated that it is desirable to control the E-beam projection such that the electrons impact the target surface in a very dense and scatter free stream of electrons. Where this is not the case and where the electrons are scattered at the time of impact onto the target surface, line features will become blurred while, for small separations between adjacent features, electrons that are meant for one area will readily impact an immediately adjacent area. This effect is known as the proximity effect. This effect can partially be eliminated by adjusting exposure density to the proximity of the elements that are being exposed which however leads to increased complexity of E-beam control and E-beam data preparation. Any method therefore that allows for the reduction of the proximity effect is an improvement in the state of the art of E-beam applications. The method of the invention allows for reducing allowable proximity from 37 nm to 26 nm.
U.S. Pat. No. 5,808,892 (Tu) shows an E-beam method that evaluates the pattern fracturing based on the figures width.
U.S. Pat. No. 5,798,528 (Butsch et al.) teaches a VSB E-beam correction method.
U.S. Pat. No. 5,217,831 (White), U.S. Pat. No. 5,13,068 (Meiri et al.), U.S. Pat. No. 5,309,354 (Dick) and U.S. Pat. No. 5,254,438 (Owen et al.) show E-beam processes that improve Critical Dimension (CD).
A principle objective of the invention is to provide a method of E-beam and laser beam exposure wherein a dummy frame figure is formed outside the main unexposed pattern.
In accordance with the objectives of the invention a new method is provided for E-beam exposure. A new method is provided for variable shaped E-beam (VSB) and Gaussian laser and E-beam exposure systems. The conventional main pattern is, under the method of the invention involving VSB, surrounded on all sides by a dummy frame whereby the dummy frame limits the beam size of the exposure shots that are adjacent to the main pattern. All patterns that are created in this manner are therefore composites using the same exposure shot. This improves the Critical Dimension (CD) uniformity of the pattern by reducing the shot linearity error for VSB exposure systems. For Gaussian beam exposure systems, the exposure shots are at times located exactly over the exposed figure. Typically, gray level is used to simulate the small figure; this however induces proximity effects. The method of the invention therefore also improves the proximity effect of the Gaussian beam exposure systems.